This invention relates to a particulate product of self-supporting spheres containing inorganic material, and more particularly relates to spheres containing particles dispersed in a gelled fugitive organic binder matrix, and also relates to a method for producing them, and to apparatus for carrying out the method.
Spherical particulates of metals and ceramics have been used or proposed for use in a variety of applications, including nuclear fuel pellets, catalysts and catalyst supports, resin and filter beds or supports, and as starting materials in the growth of single crystals from a melt. A variety of approaches have been used to produce such spheres of a desired size, size distribution, purity, etc., and in commercially economical quantities.
Many of such approaches are based upon the so-called sol-gel process, wherein the normal time for dehydration and gellation of a hydrosol of a desired inorganic particulate is considerably shortened by contacting the hydrosol droplets with an aqueous ammonia solution. See U.S. Pat. No. 2,492,808, issued to Marisic et al., Dec. 27, 1949. Subsequent refinements or improvements of this sol-gel process are represented by U.S. Pat. Nos. 3,331,783, issued to Braun et al., July 18, 1967; 3,345,437, issued to Flack et al., Oct. 3, 1967; 3,586,742, issued to Chin et al., June 22, 1971; 3,728,421, issued to Noothout Apr. 17, 1973; and 3,845,179, issued to Wace, Oct. 29, 1974.
A variation of the sol-gel process is disclosed in U.S. Pat. No. 2,746,934, issued to Richardson et al., May 22, 1956, wherein gellation is achieved by contacting the hydrosol droplets with an anhydrous aluminum alcoholate, thereby to promote dehydration of the hydrosol by hydrolysis reaction of the water from the droplets with the alcoholate to produce alcohol.
All of the above techniques depend for their success upon the formation of relatively stable but readily gellable hydrosols, and thus their applicability is limited to certain materials or material systems.
Other approaches have been taken for the production of spherical inorganic particulates. In U.S. Pat. No. 3,258,311, issued to Burzynski et al., June 28, 1966, a single continuous phase emulsion is formed and then heated to promote phase separation and the formation of spherical particles, such as silica beads. In U.S. Pat. No. 3,848,059, issued to Erneta, Nov. 12, 1974, two inorganic salts are dissolved in water droplets of two separate water-in-oil emulsions, and the emulsions are subsequently mixed together so that the salts react to form insoluble spherical precipitates.
These techniques are also somewhat limited in their applicability due to the necessity for formation of relatively stable emulsions and chemical compatibility of the starting materials.
Further approaches to the production of spherical particles include atomization of molten material into a quenching medium such as an oil bath (See, for example, U.S. Pat. No. 3,472,922, issued to Knotik, Oct. 14, 1969); spray-drying a solution or slurry of the material (See, for example, U.S. Pat. No. 3,373,119 to Krystyniak), agglomerating, and pelletizing.
Atomization of molten material into a quenching medium is a high temperature process requiring the use of special materials and handling techniques and also gives rise to special contamination problems where purity of the spherical particulates is a consideration.
Spray drying, agglomerating and pelletizing are all characterized by the production of particles rather unspherical in shape and of a wide distribution of sizings necessitating in some cases classification to obtain desired narrow size fractions. Such handling operations tend to introduce undesired contaminants in addition to those normally introduced during contact with forming materials. In addition, it is difficult to obtain particles greater than about 70 mesh (210 microns) in size by any of the above techniques.
It is therefore felt that a technique for production of spherical particulates of inorganic materials which does not depend upon the production of stable hydrosols or emulsions, or upon solubilities or compatibilities with required chemical reactions of the organic materials, or upon melting of the inorganic materials, or upon contact with contaminating media, or upon size limiting factors, would be an advancement in the art.